From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758767AbaDXSAh (ORCPT ); Thu, 24 Apr 2014 14:00:37 -0400 Received: from cantor2.suse.de ([195.135.220.15]:57312 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757849AbaDXSAg (ORCPT ); Thu, 24 Apr 2014 14:00:36 -0400 Date: Thu, 24 Apr 2014 19:00:30 +0100 From: Mel Gorman To: Dave Hansen Cc: x86@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, kirill.shutemov@linux.intel.com, ak@linux.intel.com, riel@redhat.com, alex.shi@linaro.org, dave.hansen@linux.intel.com Subject: Re: [PATCH 2/6] x86: mm: rip out complicated, out-of-date, buggy TLB flushing Message-ID: <20140424180030.GX23991@suse.de> References: <20140421182418.81CF7519@viggo.jf.intel.com> <20140421182421.DFAAD16A@viggo.jf.intel.com> <20140424084552.GQ23991@suse.de> <535942A3.3020800@sr71.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <535942A3.3020800@sr71.net> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 24, 2014 at 09:58:11AM -0700, Dave Hansen wrote: > On 04/24/2014 01:45 AM, Mel Gorman wrote: > >> +/* > >> + * See Documentation/x86/tlb.txt for details. We choose 33 > >> + * because it is large enough to cover the vast majority (at > >> + * least 95%) of allocations, and is small enough that we are > >> + * confident it will not cause too much overhead. Each single > >> + * flush is about 100 cycles, so this caps the maximum overhead > >> + * at _about_ 3,000 cycles. > >> + */ > >> +/* in units of pages */ > >> +unsigned long tlb_single_page_flush_ceiling = 1; > >> + > > > > This comment is premature. The documentation file does not exist yet and > > 33 means nothing yet. Out of curiousity though, how confident are you > > that a TLB flush is generally 100 cycles across different generations > > and manufacturers of CPUs? I'm not suggesting you change it or auto-tune > > it, am just curious. > > Yeah, the comment belongs in the later patch where I set it to 33. > > I looked at this on the last few generations of Intel CPUs. "100 > cycles" was a very general statement, and not precise at all. My laptop > averages out to 113 cycles overall, but the flushes of 25 pages averaged > 96 cycles/page while the flushes of 2 averaged 219/page. > > Those cycles include some costs of from the instrumentation as well. > > I did not test on other CPU manufacturers, but this should be pretty > easy to reproduce. I'm happy to help folks re-run it on other hardware. > > I also believe with the modalias stuff we've got in sysfs for the CPU > objects we can do this in the future with udev rules instead of > hard-coding it in the kernel. > You convinced me. Regardless of whether you move the comment or update the changelog; Acked-by: Mel Gorman -- Mel Gorman SUSE Labs